Printers for thermally recording an image or the like by giving thermal energy to a thermosensitive recording material have been developed. Particularly, printers capable of high-speed recording by using laser light as a heat source have been proposed. Further, thermosensitive recording materials including color formers, developers and light-absorbing dyes on a substrate and producing colors at a density corresponding to given thermal energy have been developed as those capable of recording good images with high quality. A printer for recording an image or the like on such a thermosensitive recording material is constructed to record a specified image by irradiating the thermosensitive recording material with laser light modulated based on an image signal.
For example, a conventional printer is known from patent literature 1. The conventional thermosensitive recording printer disclosed in patent literature 1 records an image or the like by giving specified thermal energy to a thermosensitive recording material by irradiating the thermosensitive recording material with laser light.
A control unit, an optical unit and a power supply are arranged in the above printer and power is supplied from the power supply to the control unit and the optical unit. The optical unit is controlled by the control unit in accordance with a predetermined program. Further, in the thermosensitive recording material, an information recording layer is formed on a substrate. This information recording layer is made of a material including color formers, developers and light-absorbing dyes for absorbing laser light and converting it into thermal energy.
FIG. 15 is a perspective view showing a schematic construction of the optical unit of the above conventional printer. As shown in FIG. 15, the optical unit 30 is provided with a light source 34 such as a semiconductor laser, a face tangle error correction lens 36 for causing laser light L emitted from the light source 34 to be incident on a polygon mirror 38, a long mirror 44 on which the laser light L reflected by the polygon mirror 38 is incident via a face tangle error correction lens 40 and a lens 46 for condensing the laser light L reflected by the long mirror 44. These elements are arranged in a housing 48.
The polygon mirror 38 is rotated by a motor 50 and the long mirror 44 is pivoted by a galvanometer 52, whereby the laser light L emitted from the light source 34 is scanned in a main scanning direction MS by the rotation of the polygon mirror 38 while being scanned in a sub scanning direction SS by pivotal movements of the long mirror 44.
Next, the operation of the printer constructed as above is described. The laser light L emitted from the light source 34 passes through the face tangle error correction lens 36 to be incident on the polygon mirror 38. The laser light L is scanned in the main scanning direction MS by the rotation of the polygon mirror 38, passes through the face tangle error correction lens 40 and is scanned in the sub scanning direction SS by pivotal movements of the long mirror 44. The laser light L reflected by the long mirror 44 forms circular condensed spots on a thermosensitive recording material 60 via the lens 46. The laser light L is so modulated as to record a specified gradation image by main scanning and sub scanning, and a specified gradation image is recorded on an information recording layer of the thermosensitive recording material 60 to which specified thermal energy is given by the laser light L.
In recent years, with the rapid spread of digital still cameras and the like, it has become general that individuals print photographed digital images at home or the like. At this time, it is demanded to more conveniently print by shortening a recording time (printing time).
Accordingly, in a conventional thermosensitive recording printer for scanning circular condensed spots by laser light in a main scanning direction of a thermosensitive recording material using a polygon mirror or the like to record by thermal energy as described above, speed in the main scanning direction is restricted, for example, by the rotating speed (e.g. a maximum of about 20,000 rpm) of a motor for rotating the polygon mirror. On the other hand, speed in the sub scanning direction is restricted by the feeding pitch of the condensed spots.
FIGS. 16 and 17 are diagrams showing a problem of circular condensed spots in the conventional printer. In FIGS. 16 and 17, a horizontal axis represents the main scanning direction MS and the vertical axis represents the sub scanning direction SS. As shown in FIGS. 16 and 17, in order to increase a feeding pitch P1 in the sub scanning direction SS to be equal to a feeding pitch P2 (P1<P2), condensed spots S1 of laser light condensed on the thermosensitive recording material have to be enlarged to condensed spots S2 (S1<S2) so that no region (region not scanned with the condensed spots) where recording is not performed by the feeding is produced.
However, if the condensed spots are enlarged as shown in FIG. 17, the power density of the condensed spots S2 formed on the thermosensitive recording material decreases. Thus, if the speed in the main scanning direction MS is constant and the recording sensitivity of the thermosensitive recording material is constant, the output of the light source such as a semiconductor laser has to be increased in order to make thermal energy per unit time given to the thermosensitive recording material constant, wherefore there are problems such as a power consumption increase and a cost increase.
Even if the condensed spots are enlarged, a reduction in the power density of the condensed spots formed on the thermosensitive recording material can be covered, for example, by recording the condensed spots even between pixels in an overlapping manner. FIGS. 18A and 18B are diagrams showing a state of condensed spots in the case of recording condensed spots in an overlapping manner during thermosensitive recording in the conventional printer.
Even if power density W1 of condensed spots is not sufficient when condensed spots CS are not recorded in an overlapping manner as shown in FIG. 18A, power density W2 of the condensed spots increases to increase thermal energy per unit time given to the thermosensitive recording material by scanning the thermosensitive recording material while overlapping the condensed spots CS as shown in FIG. 18B. However, in order to record a gradation image based on a specified image signal while overlapping the condensed spots CS between the pixels, the laser light needs to be modulated in consideration of the overlap of the condensed spots before and after the pixels, thereby presenting a problem of making a control very complicated.    Patent Literature 1: Japanese Unexamined Patent Publication No. H06-106761